As a system for reducing the steering effort required by a vehicle operator, or a so-called power steering system, the type, for instance, disclosed in Japanese patent publication (kokoku) No. 50-33584 is known. According to this power steering system, the steering torque applied from a steering wheel is assisted by an output torque of an electric motor. By varying the gain for the detected signal corresponding to the steering torque applied to the steering wheel by a vehicle operator according to the vehicle speed and the road condition, the output torque of the assisting electric motor is adjusted so that an optimum steering torque may be obtained at all times.
When a vehicle encounters a strong crosswind or side wind, the vehicle tends to deviate from the intended straight path of travel. To maintain the vehicle on the straight path, it is necessary to apply a reaction to the steerable wheels which opposes such an external disturbance.
According to the conventional power steering system, however, the assisting torque is produced only when the vehicle operator has applied some steering torque to the steering wheel. Therefore, when the vehicle deviates from the intended straight path of travel due to crosswind, the electric motor will not produce any assisting torque by itself.
Therefore, to prevent the deviation of the vehicle from the intended path, the vehicle operator must constantly apply a manual steering torque to the steering wheel. According to the conventional steering system, the assisting torque is generally small when the lateral acceleration and the yaw rate of the vehicle are large. Therefore, when the vehicle deviates from an intended path of travel, the steering input necessary to restore the vehicle to the intended path of travel becomes progressively greater as the lateral acceleration and the yaw rate of the vehicle increase.
Thus, to overcome this inconvenience, it has been proposed to detect an irregular behavior of the vehicle caused by an external disturbance from the yaw rate or lateral acceleration of the vehicle, and to produce a reaction with an electric motor for producing a steering torque which cancels the effect of such a disturbance so that the irregular behavior of the vehicle due to external disturbances may be controlled.
However, according to such a control system using an electric motor, for instance, if the lateral acceleration sensor, the yaw rate sensor or the reaction control device should fail, there would be no steering torque as it is normal to stop the electric motor in such a situation, and this would not only cause discomfort to the vehicle operator but also impair the convenience of the steering system.